1. Field of the Invention
The present invention relates to a latent-image measuring device, and a latent-image carrier.
2. Description of the Related Art
For example, Japanese Patent Application Laid-open No. H03-49143 has proposed a conventional method of observing an electrostatic latent image using electron beams. In the conventional method, however, samples for use in forming an electrostatic latent image thereon are limited to large-scale-integration (LSI) chips and those that can store and hold an electrostatic latent image. That is, an ordinary photoconductor having dark attenuation characteristics cannot be used as a sample. Because the ordinary dielectric substance can semipermanently hold an electric charge, when an electric charge distribution is measured after its formation, there is no influence in the measurement result. On the other hand, in the case of the photoconductor, because the resistance value is not limitless, the photoconductor cannot hold an electric charge for a long time, and the surface potential decreases along lapse of time due to the dark attenuation characteristics. That is, the photoconductor holds an electric charge for only some seconds in a dark room. Therefore, even when an electrostatic latent image is attempted to be observed with a scanning electron microscope (SEM) after charging and exposure, the electrostatic latent image disappears at the preparation stage.
To overcome the problem, for example, Japanese Patent Application Laid-open No. H2003-295696 has proposed a method of measuring an electrostatic latent image formed on a photoconductor as a sample having dark attenuation.
FIGS. 20A and 20B are schematic diagrams for explaining charge-distribution/potential-distribution detection using secondary electrons.
When charge distribution is present on the surface of the sample, an electric field distribution is spatially formed according to the surface charge distribution. Therefore, generated secondary electrons e are pushed back by incident electrons (not shown), and the amount of the secondary electrons e that reach a detector S decreases.
In the example of FIGS. 20A and 20B, light is irradiated onto a point on the surface of the photoconductor sample that is uniformly negatively charged with a charge density Q, and thus, charge density locally decreases. For example, secondary electrons e1 and e2 generated by incident electrons that reach a position P1 and a position P2 are pulled in a direction of the detector S forming an antipole, and reach the detector S drawing flight trajectories G1 and G2, respectively. On the other hand, when an incident electron reaches a position indicated by P3, for example, a generated secondary electron e3 once flies out from the surface of the sample. However, because a relatively opposite direction of an electric field distribution is formed at this position, force is generated toward the surface of the sample to return the secondary electron e3 back as indicated by a flight trajectory G3. Therefore, the secondary electron e3 is absorbed into the surface of the sample, and does not reach the detector S.
Accordingly, a part of the surface of the sample having strong electric field intensity is dark, and a part having weak electric field intensity is bright, generating a contrast of brightness. As a result, a contrast image according to the surface charge distribution can be detected. Consequently, when the surface is exposed, the exposed part becomes black, and a non-exposed part becomes white, thereby forming an electrostatic latent image which can be measured.
FIG. 21 is a chart for explaining reciprocity failure.
A photoconductor has the occurrence of a phenomenon of reciprocity failure that even when the total exposure energy given to the photoconductor is the same, when a relation between the light intensity and the exposure time is different, a latent-image formation state is different. In general, when the exposure energy is constant, and when light intensity is strong (when the exposure time is short), sensitivity (depth of the latent image) decreases. As a result, image density is differentiated. This is considered to occur because of a phenomenon that when the light intensity is strong, the amount of recombined carriers increases, and the amount of carriers reaching the surface decreases. When a multi-beam scanning optical system is used, this phenomenon remarkably appears as uneven image density.
FIG. 22 is a schematic diagram for explaining image density when a 4-ch LD array is used as the scanning optical system of the image forming apparatus.
A boundary area of LD1 and LD2 is exposed substantially simultaneously. Therefore, large amount of light is applied to this area in a short time. On the other hand, in a boundary area of LD4 and LD1, LD4 is exposed first, and then LD1 is exposed. Therefore, a time difference occurs, and weak light intensity is applied during a long time as a result. In this case, when exposure is carried out in a long delay time, a deep latent-image potential distribution is formed, and a toner can easily adhere to the surface. As a result, the boundary area of LD4 and LD1 has high image density, and uneven image density occurs.
FIG. 23 is an enlarged view of a photoconductor surface. The phenomenon of reciprocity failure depends on a charge-generation-layer (CGL) film thickness, carrier mobility, quantum efficiency, and a generation amount of carriers, among characteristics of a photoconductor. Therefore, it is desirable to provide an imaging system including a photoconductor that does not easily generate reciprocity failure, and a scanning optical system. However, according to the conventional measuring method, space resolution of only about a few millimeters can be obtained, and sufficient precision cannot be obtained to analyze the mechanism.
When charge distribution is present on the surface of the sample, an electric field distribution according to the surface charge distribution is spatially formed. Therefore, a secondary electron generated by the incident electron is returned back by this electric field, and the amount of the secondary electrons reaching the detector decreases. Accordingly, a part having strong light intensity is dark, and a part having weak light intensity is bright, thereby generating a contrast of brightness, making it possible to detect a contrast image according to the surface charge distribution. Therefore, when light is irradiated, the exposed part becomes black, and the non-exposed part becomes white, thereby making it possible to measure an electrostatic latent image formed (see FIGS. 20A and 20B).
A photoconductor has the occurrence of a phenomenon of reciprocity failure that even when the total exposure energy given to the photoconductor is the same, when a relation between the light intensity and the exposure time is different, a latent-image formation state is different. In general, when the exposure energy is constant, and when light intensity is strong, sensitivity (depth of the latent image) decreases. As a result, image density is differentiated (see FIG. 21). This is considered to occur because of the phenomenon that when the light intensity is strong, the amount of recombined carriers increases, and the amount of carriers reaching the surface decreases. When a multi-beam scanning optical system is used, this phenomenon conspicuously appears as uneven image density.
The phenomenon of reciprocity failure depends on a CGL film thickness, carrier mobility, quantum efficiency, and the generation amount of carriers, among characteristics of a photoconductor. Therefore, it is desirable to provide an imaging system including a photoconductor that does not easily generate reciprocity failure, and a scanning optical system. However, according to the conventional measuring method, space resolution of only about a few millimeters can be obtained, and sufficient precision cannot be obtained to analyze the mechanism.
Furthermore, mobility of carriers generated by laser beams or by light-emitting-diode (LED) exposure is calculated using a time-of-flight method for calculating a time in which a carrier generated at the sample side moves to the opposite surface. However, the CGL is a thin film of about 0.1 micrometer, and cannot obtain sufficient precision. A moving speed and a moving time of a carrier give large influence to the formation of a latent image, and are very important factors to obtain high image quality.